Breast cancer is a major health concern, with over 200,000 new diagnoses rendered each year in the United States. Approximately 1 in 8 women will develop breast cancer; thus, substantial effort has been directed at defining the basis of tumor development and progression. Breast cancer is represented by multiple disease- subtypes which are distinguished by differential markers, prognoses, and treatment regimens. In general, estrogen receptor, progesterone receptor and Her 2 negative disease (ie. triple negative breast cancer-TNBC) is faster progressing and more difficult to treat. Genetic analyses have established that specific tumor suppressor pathways are differentially disrupted in ER-negative disease, but the specific relevance of these events for tumor behavior or therapeutic response remains unclear. Here, we will delineate the coordinate role of RB-tumor suppressive pathway in the progression to ER-negative disease and the treatment of such tumors based on rational drug delivery. Two major areas of preliminary investigation support the rationale and hypothesis of the proposal: First, preliminary and independently published data demonstrate that loss of RB gene, and/or inactivation of the RB-pathway occurs at high frequency in basal ER-negative breast cancer. Supporting this contention, preliminary analyses of ductal carcinoma in situ lesions demonstrate that RB- pathway alterations are observed early in ER-negative disease. Furthermore, models of RB deletion exhibit molecular signatures indicative of basal ER-negative breast cancer. Thus, we will characterize the impact of RB-pathway on the behavior of pre-invasive lesions, underlying prognosis, and additional genetic events associated with progression to invasive cancer (Aim 1). Second, we observed in multiple preclinical models that loss of RB increases sensitivity to cytotoxic therapies. Subsequent independent analyses of ER-negative clinical specimens demonstrated that loss of RB is associated with an improved response to conventional cytotoxic chemotherapy. Given the frequency of RB loss in human disease, it is essential to define the impact of RB status for the molecular and cellular consequence response to therapy and define new regimens to specifically exploit the vulnerabilities encoded with the loss of RB (Aim 2). Collectively, the studies proposed herein will test the hypothesis that the RB-pathway plays a critical role in modulating disease progression and therapeutic response in triple negative breast cancer that could be rationally targeted.